In the semiconductor industry, photolithographic exposure tools such as steppers and scanners have been used to define patterns in photosensitive material known as photoresist. After photoresist material is spun onto a substrate, an exposure tool repeatedly projects an image of the pattern to be defined on the mask to repeatedly expose the photoresist layer. The properties of the exposed portions of the photoresist layer are altered for subsequent processing steps such as resist development and consecutive substrate etching or implantation.
A mask is typically a transparent plate such as quartz with opaque elements such as a chrome layer on the plate used to define a pattern. A radiation source illuminates the mask according to well-known methods. The radiation transmitted through the mask and exposure tool projection optics forms a diffraction-limited latent image of the mask features on the photoresist layer. Further discussion of patterning principles and diffraction limited microlithography can be found on pages 274-276 of VLSI Technology edited by S. M. Sze (® 1983).
However, because of increased semiconductor device complexity, which results in increased pattern complexity, increased resolution demands, and increased pattern packing density on the mask, the distance between any two opaque areas has decreased. By decreasing the distances between the opaque areas, small apertures are formed which diffract the light that passes through the apertures. The diffracted light results in effects that tend to spread or to bend the light as it passes so that the space between the two opaque areas is not resolved, therefore making diffraction a severely limiting factor for conventional optical lithography.
As feature sizes decrease, semiconductor devices are typically less expensive to manufacture and demonstrate higher performance. In order to produce smaller feature sizes, an exposure tool having adequate resolution and depth of focus at least as deep as the thickness of the photoresist layer is desired. For exposure tools that use conventional or oblique illumination, better resolution can be achieved by lowering the wavelength of the exposing radiation or by increasing the numerical aperture of the exposure tool, but the smaller resolution gained by increasing the numerical aperture is typically at the expense of a decrease in the depth of focus for minimally resolved features. This constraint presents a difficult problem in reducing the patterning resolution for a given radiation wavelength.
One method of printing smaller features with smaller critical dimensions while maintaining a sufficient depth of focus involves the use of a phase shift mask (PSM). A PSM uses phase shift layers, which shift the phase of the incident radiation to transmit radiation 180 degrees out of phase compared to radiation transmitted by adjacent non-shifted layers. The radiation transmitted by the phase shift layers destructively interferes with radiation transmitted by adjacent non-shifted layers in the areas of the image plane most susceptible to depth of focus limitations. How to further improving the resolution and depth of focus limitations is an exigent object to be overcome.